Britten, a pacifist, was inspired by the commission, which gave him complete freedom in deciding what to compose. He chose to set the traditional Latin Mass for the Dead interwoven with nine poems about war by the English poet Wilfred Owen. Owen, who was born in 1893, was serving as the commander of a rifle company when he was killed in action on 4 November 1918 during the crossing of the Sambre-Oise Canal in France, just one week before the Armistice. Although he was virtually unknown at the time of his death, he has subsequently come to be revered as one of the great war poets.

Philip Reed has discussed the progression of Britten's composition of the War Requiem in the Cambridge Music Handbook publication on the work.[3] Britten himself acknowledged the stylistic influence of Requiems by other composers, such as Giuseppe Verdi's, on his own composition.[4]

Britten dedicated the work to Roger Burney, Piers Dunkerley, David Gill, and Michael Halliday. Burney and Halliday, who died in the war, were friends of Peter Pears and Britten, respectively. According to the Britten-Pears Foundation's War Requiem website, Dunkerley, one of Britten's closest friends, took part in the 1944 Normandy landings. Unlike the other dedicatees, he survived the war but committed suicide in June 1959, two months before his wedding. None of the other dedicatees have known graves, but are commemorated on memorials to the missing.[5]

The musical forces are divided into three groups that alternate and interact with each other throughout the piece, finally fully combining at the end of the last movement. The soprano soloist and choir are accompanied by the full orchestra, the baritone and tenor soloists are accompanied by the chamber orchestra, and the boys' choir is accompanied by a small positive organ (this last group ideally being situated at some distance from the full orchestra). This group produces a very strange, distant sound. The soprano and choir and the boys' choir sing the traditional Latin Requiem text, while the tenor and baritone sing poems by Wilfred Owen, interspersed throughout.

The interval of a tritone between C and F♯ is a recurring motif, the occurrence of which unifies the entire work. The interval is used both in contexts that emphasise the harmonic distance between C and F♯ and those that resolve them harmonically, mirroring the theme of conflict and reconciliation present throughout the work.[6] The Requiem aeternam, Dies irae, and Libera me movements end in a brief choral phrase, consisting mainly of slow half notes, each first and second phrase ending on a tritone's discord, with every last (i. e. third) phrase resolving to an F major chord; while at the end of the Agnus Dei the tenor (in his only transition from the Owen poems to the Requiem liturgy, on the key words, Dona nobis pacem – Give us peace) outlines a perfect fifth from C to G before moving down to F♯ to resolve the chorus's final chord. At the end of the Dies irae, the tenor sings (from Owen's "Futility") "O what, what made fatuous sunbeams toil, to break earth's sleep at all?" The notes of "at all" form the tritone and lead into the choir's formal resolution. In the final Owen setting, "Strange Meeting", one of the most prominent expressions of the tritone is sung by the tenor, addressing an opposing soldier with the words "Strange friend". This poem is accompanied by sporadic detached chords from two violins and a viola, which include the tritone as part of a dominant 7th chord. At the end of the poem, the final string chord resolves to the tonic, bringing the work to its final, reconciliatory In paradisum. On a more practical level, Britten facilitated musical execution of the tritone in the closing bars by having the F# sung in one voice, but the C in another.[7]

Four other motifs that usually occur together are distinct brass fanfares of the Dies irae: a rising arpeggio, a falling arpeggio followed by a repeated note, a repeated fourth in a dotted rhythm ending in a diminished arpeggio, and a descending scale. These motifs form a substantial part of the melodic material of the piece: the setting of "Bugles sang" is composed almost entirely of variations of them.

Another linking feature can be found in the opening of the final movement, Libera Me, where the slow march tune in the double basses (preceded by two drums outlining the rhythm) replicates the more-rapid opening theme of the first poem, Anthem for Doomed Youth.

One striking juxtaposition is found in the Offertorium, a fugue in the repeating three-part-time scheme 6/8, 9/8, 6/8 where the choir sings of God's promise to Abraham ("Quam olim Abrahae promisisti, et semini eius" – "which you once promised Abraham and his seed"). This frames Owen's retelling of the offering of Isaac, in which the angel tells Abraham to:

'... offer the ram of pride instead of him.'

But the old man would not so, but slew his son,
and half the seed of Europe, one by one.

As the male soloists sing the last line repeatedly, the boys sing "Hostias et preces tibi, Domine" ("Sacrifice and prayers we offer thee, Lord"), paralleling the sacrifice of the Mass with the sacrifice of "half the seed of Europe" (a reference to World War I). The "reprise" of "Quam olim Abrahae" is sung in inversion, diminuendo instead of crescendo.

The whole of the Offertorium is a reference to Britten's earlier Canticle No. 2 "Abraham and Isaac" from 1952. Britten here uses much of the musical material of the earlier work, but the music in the Requiem is twisted into much more sinister forms.

Although there are a few occasions in which members of one orchestra join the other, the full forces do not join together until the latter part of the last movement, when the tenor and baritone sing the final line of Owen's poem "Strange Meeting" ("Let us sleep now ...") as "In Paradisum deducant" ("Into Paradise lead them ...") is sung first by the boys' choir, then by the full choir (in 8-part canon), and finally by the soprano. The boys' choir echoes the Requiem aeternam from the beginning of the work, and the full choir ends on the resolved tritone motif.

For the opening performance, it was intended that the soloists should be Galina Vishnevskaya (a Russian), Peter Pears (an Englishman) and Dietrich Fischer-Dieskau[8][9] (a German), to demonstrate a spirit of unity. Close to the premiere, the Soviet authorities did not permit Vishnevskaya to travel to Coventry for the event,[10] although she was later permitted to leave to make the recording in London. With only ten days' notice, Heather Harper stepped in and performed the soprano role.

Although the Coventry Cathedral Festival Committee had hoped Britten would to be the sole conductor for the work's premiere, shoulder pain forced his withdrawal from the main conducting role.[11] He did, however, conduct the chamber orchestra, and this spawned a tradition of separate conductors that the work does not require and Britten never envisaged.[12] The premiere took place on 30 May 1962, in the rebuilt cathedral with the City of Birmingham Symphony Orchestra, conducted by Meredith Davies[13] (accompanying soprano and chorus), and the Melos Ensemble, conducted by the composer (accompanying tenor and baritone).[14] At Britten's request, there was no applause following the performance.[15] It was a triumph, and critics and audiences at this and subsequent performances in London and abroad hailed it as a contemporary masterpiece.[16] Writing to his sister after the premiere, Britten said of his music, "I hope it'll make people think a bit." On the title page of the score he quoted Wilfred Owen:

To commemorate the eve of the 70th anniversary of the destruction of the original cathedral, a performance of the Requiem took place in the new cathedral on 17 November 2010, featuring the soprano Claire Rutter, the tenor Daniel Norman, baritone Stephen Gadd, The Parliament Choir, Saint Michael's singers, Deutscher Chor London, the ESO Chamber Orchestra, The Southbank Sinfonia and The Girl Choristers of Coventry Cathedral. It was conducted by Simon Over and Paul Leddington Wright. A recording was made and broadcast a day later on Classic FM.

A 50th anniversary performance was given by the City of Birmingham Symphony Orchestra conducted by Andris Nelsons at Coventry Cathedral on 30 May 2012.[18]

The first recording, featuring Vishnevskaya, Fischer-Dieskau and Pears, with the London Symphony Orchestra and The Bach Choir conducted by Britten, was produced by Decca in 1963. Within five months of its release it sold 200,000 copies, an unheard-of number for a piece of contemporary classical music at that time.[20] Recording producer John Culshaw reports that Vishnevskaya threw a tantrum during the recording, as she believed – not having performed the work before – she was being insulted by being placed with the choir instead of at the front with the male soloists.[21][22] The newest (2013) CD reissue of this recording includes 50 minutes of surreptitiously taped rehearsal footage at the time of the recording.

In 1988, the British film director Derek Jarman made a screen adaptation of War Requiem of the same title, with the 1963 recording as the soundtrack, produced by Don Boyd and financed by the BBC. It features the final film performance of Laurence Olivier, in the role of an ageing war veteran.